Photovoltaic cells where light is converted into electric power to be fed to external loads, which are electrically connected to the photovoltaic cells, have been prevailing in a wide range of applications such as consumer electronics, industrial electronics, and space exploration. In consumer electronics, photovoltaic cells that consist of materials such as amorphous silicon are used for a variety of inexpensive and low power applications. Typical conversion efficiency, i.e. the solar cell conversion efficiency, of amorphous silicon based photovoltaic cells is in the range of ˜10% [Yamamoto K, Yoshimi M, Suzuki T, Tawada Y, Okamoto T, Nakajima A. Thin film poly-Si solar cell on glass substrate fabricated at low temperature. Presented at MRS Spring Meeting, San Francisco, April 1998.]. Although the fabrication processes of amorphous silicon based photovoltaic cells are rather simple and inexpensive, one notable downside of this type of cell is its vulnerability to defect-induced degradation that decreases its conversion efficiency.
In contrast, for more demanding applications such as residential and industrial solar power generation systems, either poly-crystalline or single-crystalline silicon is typically used because there are more stringent requirements of better reliability and higher efficiency than those in consumer electronics. Photovoltaic cells consisting of poly-crystalline and single-crystalline silicon generally offer conversion efficiencies in the range of ˜20% and ˜25% [Zhao J, Wang A, Green M, Ferrazza F. Novel 19.8% efficient ‘honeycomb’textured multicrystalline and 24.4% monocrystalline silicon solar cell. Applied Physics Letters 1998; 73: 1991-1993.] respectively. As many concerns associated with a steep increase in the amount of the worldwide energy consumption are raised, further development in industrial solar power generation systems has been recognized as a main focus for an alternative energy source. However, due to the high cost ($3 to $5/Watt) of today's Si-based solar cell, it is not yet widely accepted as an alternative energy source solution.
Group II-VI compound semiconductors, for example CdTe and CdS, have been considered for the purpose of creating industrial solar power generation systems, manufactured at a lower cost and maintaining a moderate conversion efficiency. This approach resulted in a comparable conversion efficiency of ˜17% [Wu X, Keane J C, Dhere R G, DeHart C, Duda A, Gessert T A, Asher S, Levi D H, Sheldon P. 16.5%-efficient CdS/CdTe polycrystalline thin-film solar cell. Proceedings of the 17th European Photovoltaic Solar Energy Conference, Munich, 22-26 Oct. 2001, 995-1000.]. This conversion efficiency is comparable to those for the single crystalline silicon photovoltaic devises; however, the toxic nature of these materials is of great concern for environment.
Group I-III-VI compound semiconductors, such as CulnGaSe2, have also been extensively investigated for industrial solar power generation systems. This material can potentially be synthesized at a much lower cost than its counterpart, single crystalline silicon. However, a conversion efficiency of ˜19%, is comparable to that of single crystalline silicon based cells and can be obtained, thus far, only by combining with the group II-VI compound semiconductor cells [Contreras MA, Egaas B, Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, Noufi R. Progress toward 20% efficiency in Cu(In,Ga)Se polycrystalline thin-film solar cell. Progress in Photovoltaics: Research and Applications 1999, 7: 311-316.], which again raises issues associated with the toxic nature of these materials.
Photovoltaic cells designed for several exclusive applications, where the main focus is high conversion efficiency, generally consist of group III-V semiconductors, including GalnP and GaAs. In general, synthesis processes of single crystalline group III-V are very costly because of substantial complications involved in epitaxial growth of group III-V single crystalline compound semiconductors. Typical conversion efficiencies of group III-V compound semiconductor based photovoltaic cells, as these types of photovoltaic cells are intended to be, can be as high as ˜34% when combined with germanium substrates, another very expensive material [King R R, Fetzer C M, Colter P C, Edmondson K M, Law D C, Stavrides A P, Yoon H, Kinsey G S, Cotal H L, Ermer J H, Sherif R A, Karam N H. Lattice-matched and metamorphic GalnP/GalnAs/Ge concentrator solar cells. Proceedings of the World Conference on Photovoltaic Energy Conversion (WCPEC-3), Osaka, May 2003, to be published.], usually more than 10 times as expensive as the conventional Si-solar cell.
All photovoltaic cells in the prior art described above, regardless of what materials the cell is made from, essentially fall into one specific type of structure, which usually limits its power generation capability. Usually a flat pn-junction structure is used in conventional solar cells (FIG. 1A). Shown in FIG. 1A is a photovoltaic cell comprising a thick p-type semiconductor layer 101 and a thin n-type semiconductor layer 102 formed on an electrically conductive substrate 100. A pn-junction 103 is formed at the interface between the p-type semiconductor layer 101 and the n-type semiconductor layer 102. Incident light 104 entering the cell generates electron-hole pairs after being absorbed by the p- and also n-type semiconductor layers 101 and 102. The incident light 104 generates electrons 105e and holes 105h in the region near the pn-junction 103 and also electrons 106e and holes 106h in the region far from the pn-junction 103. The photogenerated electrons 105e and 106e (and holes) (hereafter considering only electronics, i.e. minority carriers in p-type semiconductors, although the same explanation is applicable for holes, minority carriers in n-type semiconductors) diffusing toward the pn-junction 103 and entering the pn-junction 103 contribute to photovoltaic effect. The two key factors that substantially impact the conversion efficiency of this type of photovoltaic cell are photo carrier generation efficiency (PCGE) and photo carrier collection efficiency (PCCE).
The PCGE is the percentage of photons entering a cell which contribute to the generation of photo carriers, which needs to be, ideally, 100%. On the other hand, the PCCE is the percentage of photogenerated electrons 105e and 106e that reach the pn-junction 103 and contribute to the generation of photocurrent. For a monochromatic light, a PCGE of ˜100% can be achieved by simply making the p-type layer 101 thicker; however, electrons 106e generated at the region far away from the pn-junction 103 cannot be collected efficiently due to many adverse recombination processes that prevent photogenerated carriers from diffusing into the pn-junction 103. Thus, the basic structure of current photovoltaic cells has its own limitation on increasing the conversion efficiency. As the minority carriers travel through the semiconductors, the longer the life-time, the less recombination, which increases the conversion efficiency. Usually, a thicker and higher quality wafer is used to increase the conversion efficiency of the conventional solar cell. However, this makes the solar cell costly and heavier. In addition to increasing the collection efficiency, the absorption of a wide range of the solar spectrum will also increase the conversion efficiency. It is highly desirable to have the solar cell structure in which (a) the increase of the PCCE is independent of the substrate thickness and (b) the ability to absorb a wide range of the solar spectrum is possible.
FIG. 1B shows typical monochromatic light intensity behavior 108 inside the semiconductor. As illustrated in FIG. 1B, the light intensity behavior 108 inside the bulk semiconductor is exponential. The light intensity p at certain depth x can be expressed as p(x)=Poexp(−αx), where Po is the peak intensity at the surface and α is the absorption co-efficient of the semiconductor in which light is entering. Carriers (not shown here) generated due to light flux 112 absorbed by the pn-junction 103 is only drifted by the junction field and can be collected efficiently, whereas, carriers 106e and 106h generated due to absorption of light-flux 110 by semiconductor region 101 are diffused in all directions. Only those carriers 105e and 105h which are generated closer (a distance equal to or less than the diffusion-length of the semiconductor) to the pn-junction 103, can be collected. Those carriers 106e and 106h which are generated far away (a distance longer than the diffusion-length of the semiconductor) from the pn-junction 103 are recombined and lost. The light flux 112 is usually lost either by leaving out or being absorbed by the substrate.
Both PCGE and PCCE are largely dependent on the material and structure of the photovoltaic cells. Today's photovoltaic cells are structured in such a way that (a) wide ranges of the solar spectrum cannot be absorbed due to material limitations, and (b) PCCE is low due to its inherent structure. For example, the typical conversion efficiency of today's crystal-Si based solar cell is ˜18%. Wavelengths of the solar spectrum spread from <0.1 μm to 3.5 μm, but Si can only absorb ˜0.4 μm to 0.9 μm of light. ˜50% of light belonging to the solar spectrum cannot be absorbed by Si, due to its inherent material properties. The remaining 32% is lost due to (i) recombination of photogenerated carriers and (ii) loss of light, which is represented by 112 in FIG. 1B; these two factors are structure dependent. If we could reduce these two factors, ˜50% conversion efficiency could be achieved, even in a Si-based solar cell. If we could capture different wavelengths of light belonging to the solar spectrum by utilizing different material systems or nano-material systems, we could increase the conversion efficiency ideally to 100%. Furthermore, if the solar cell (photovoltaic cell) detection capability could be extended to the infrared-spectrum, then the cell could produce electrical energy not only during the day (while sun is present), but also at night (hereafter defined by when the sun is not out). Additionally, today's solar cell material is not highly radiation-tolerant. Specifically, in space applications, photovoltaic cells should be highly radiation tolerant and have structure and material systems which can generate high-power per unit area. In order to increase the conversion efficiency (ideally to 100%), it would be desirable to have photovoltaic cell structures which have (a) larger surface area to volume ratios to capture all the photons (at specific wavelength) entering the cell, (b) a pn-junction that is located as close to the photo absorption region as possible, and (c) photo responses at different spectrums in order to efficiently cover a wide range of spectrums of light that enter a photovoltaic cell. It would be further desirable to have solar cells which could generate electric power in both day and night.
In addition to conversion efficiency, cost-effective manufacturing is another important factor which needs to be taken into consideration. In today's solar cell, the high-cost is one of the main concerns in addition to the issue of achieving low conversion efficiencies. It is found that more than 93% of solar cells are silicon (Si) based, meaning a silicon (Si) wafer is the base material, and the rest are thin-film based solar cells. In manufacturing Si-based solar cells, more than 85% of the cost comes from the Si wafer cost, the remaining comes from other processing costs. It is highly desirable to reduce the wafer cost and at the same time increase the conversion efficiency.